Bipolar transistors are well known in the semiconductor arts. Many bipolar transistors include a doped tub having a base region of the opposite conductivity type formed therein. An emitter region of the same conductivity type as the doped tub is formed in the doped base region. The doped tub serves as a collector. An emitter electrode contacts the emitter region directly above the doped tub while the base contact is laterally removed from the base region itself to minimize size of the base region and improve device performance. The collector electrode usually couples to a buried layer which is heavily doped the same conductivity type as the tub and is positioned underneath the tub.
One problem with conventional bipolar transistors is that the diffused junctions require high temperature diffusion cycles which makes formation of bipolar transistors incompatible with field effect devices, which normally required low temperature processing. Another problem is that collector and emitter electrodes are not interchangeable, complicating layout of integrated circuits using the devices. Still another difficulty is that device characteristics are highly sensitive to base width, which is determined by process control of both photolithography process steps and diffusion process steps. Diffusion steps in particular are difficult to control when making transistors with sub-micrometer features.
Another problem with bipolar transistors is that base resistance is high in this type of structure because the active base is not directly contacted. The high base resistance inhibits frequency and AC performance. Further, the base region must be large to have an emitter region formed therein and still function properly. This large base region requires increased device size, causes out-diffusion problems and creates high junction capacitance. Because the active area is large, the base to substrate capacitance is also large. The large base width causes transit time through the base region to be large.
A partial solution to these problems is provided by lateral bipolar transistors which include a central base region and emitter and collector regions formed on either side of the base region. All of the device regions of a lateral bipolar transistor can be contacted directly from the top surface of the device. Lateral bipolar transistors require deep junctions, however, to achieve acceptable performance. Deep junctions result in large geometry devices because of lateral diffusion. Until now, lateral bipolar transistors were either too large to be practical, or severely compromised electrical characteristics such as gain and current carrying capability.
In view of the above, it would be highly desirable to have a bipolar transistor structure that improves upon the above-identified problems encountered by standard bipolar transistors.